Communication systems allow users to communicate with each other over a network. The network may be, for example, the internet or the Public Switched Telephone Network (PSTN). Audio signals can be transmitted between nodes of the network, to thereby allow users to transmit and receive audio data (such as speech data) to each other in a communication session over the communication system.
A device may have audio output means such as speakers for outputting audio signals to the user. The audio signals that are output from the speakers may be received over the network from the user(s) during the call. The speakers may also be used to output audio signals from other audio sources. For example, the device may be a TV in which case the speakers will also output TV audio signals.
The device may also have audio input means such as a microphone that can be used to receive audio signals, such as speech from a user. The user may enter into a communication session with another user, such as a private call (with just two users in the call) or a conference call (with more than two users in the call). The user's speech is received at the microphone, processed and is then transmitted over a network to the other user(s) in the call.
Acoustic echo occurs when the audio signals that are output from a speaker are picked up by a microphone. Techniques to solve this problem are known as acoustic echo cancellation (AEC) methods. Very high amplification in the echo path is a source of severe AEC problems on hands-free devices including TVs. The spectrum of audio signals received at the microphone may be dominated by certain narrow parts where the echo path has a high gain, also known as a resonating frequency. If the echo cancellers on both sides have leakage it gives rise to a phenomenon known as feedback, or howling. When calling a device with similar characteristics feedback can easily occur as both sides will amplify the most in the same part of the spectrum. This can be a severe problem and will interfere with the communication and in some cases render the call useless.
In order to reduce the problems of acoustic echo, microphones are often placed as far away as possible from the speakers. For example a microphone may be placed in the middle of the speakers, or in a corner of the device. This changes the echo path for the better but there will remain frequencies which have high echo path gain causing disturbance during a call.
The use of stereo microphones and microphone arrays in which a plurality of microphones operate as a single device are becoming more common. These enable the use of extracted spatial information in addition to what can be achieved in a single microphone. When using such devices one approach to suppress echo is to apply a beamformer at the microphone. Microphone beamforming is the process of trying to focus the signals received by the microphone array by applying signal processing to enhance sounds coming from one or more desired directions. This may be achieved by estimating the angle (and the distance) from which wanted signals are received at the microphone array. This so-called Direction of Arrival (“DOA”) information can be determined or set prior to the beamforming process. In alternative situations it can be advantageous to adapt the desired direction of arrival to changing conditions, and so it may be advantageous to perform the estimation of the desired direction of arrival in real-time as the beamformer is used. Adaptive beamformers apply a number of weights (or “beamformer coefficients”) to the received audio signals. These weights can be adapted to take into account the DOA information to process the audio signals received by the plurality of microphones to form a “beam” whereby a high gain is applied to desired audio signals received by the microphones from a desired location (i.e. a desired direction and distance) and a low gain is applied in the directions to any other (e.g. interfering) signal sources.
While the microphone beamformer will attempt to suppress the echo coming from unwanted directions, the number of microphones as well as the shape and the size of the microphone array will limit the effect of the beamformer, and as a result the unwanted echo audio signals are suppressed, but remain audible.